This invention is directed at an improved extractive separation process. More specifically, the present invention is directed at an improved method for directing feed and/or recycle streams into an extraction zone to improve the extraction zone performance.
In the separation of a mixture of components, one or more unit operations may be utilized. The particular unit operation normally is selected after a consideration of several variables, including the desired purity of the separated fractions, capital equipment cost, equipment reliability and operating costs. In many separation operations, extractive separation may be the preferred method of separation. Typically, a solvent having the desired affinity for one or more of the feed and/or recycle stream components is passed through the extraction zone countercurrently to the feed and/or recycle streams to effect the separation. For example, in the separation of aromatics, such as benzene, toluene, and xylene (BTX) from non-aromatics, such as hexane, heptane, and isoheptane, the feed and/or recycle stream may be contacted with a solvent such as sulfolane (tetrahydrothiophene-1,1-dioxide), tetraethylene glycol, phenol, dimethysulfoxide, furfural, or N-methyl pyrrolidone. In the preparation of a lube oil basestock from a hydrocarbon feedstock the hydrocarbon feedstock may be extracted with a solvent such as phenol, N-methyl pyrrolidone and furfural.
In the conventional process, the solvent typically is introduced substantially at the top of the extraction zone, flows downwardly, and exits at the bottom enriched with the extracted component. The hydrocarbon inlet feedstream typically is introduced at a single location at or near the bottom of the extraction zone, flows upwardly, and exits near the top of the extraction zone with a reduced concentration of the extracted component. The inlet recycle stream, if present, typically is introduced at a single location below the hydrocarbon feed location to achieve a backwashing effect for displacing certain components from the solvent before the solvent exits from the base of the extractor. In the extractive separation of aromatics from non-aromatics, recycle stream components may displace from the aromatic-rich solvent into the raffinate certain non-aromatic components which would be more difficult to remove from the BTX product in downstream purification steps. This recycle stream typically may be a product stream from a downstream purification stage which removes non-aromatics from the aromatic products.
In many extractive operations, it may be desired to improve the capacity and/or separation achieved in the extraction zone of new or existing extractive separation zones without the need for additional processing equipment and without increasing the size and/or operating costs. In other operations, upstream modifications or changes in feeds may have altered the composition or flow rates to the extraction zone from those utilized in the original design of the extraction zone.
Accordingly, it is desirable to provide a reliable, flexible process which may even be retrofitted to existing extraction zones to permit increased throughput rates without adversely affecting product quality and without causing a significant increase in operating costs.
It also is desirable to provide a process which may improve the feed separation in an extraction zone.
The subject invention is directed at the discovery that the efficiency of an extractive separation process may be improved by passing the feedstream and/or recycle stream into the extraction zone simultaneously through a plurality of spaced-apart nozzles rather than through a single nozzle. It also has been discovered that the capacity and/or separation efficiency of an extractive separation in which a recycle stream from a downstream product purification step is recycled to the extraction zone may be improved if at least a portion of the recycle is added to the extraction zone at a location above the location at which all the feed is added to the extraction zone. This improvement in extraction zone efficiency permits an increase in product quality or through-put.